ARCHAEOLOGICALCONSERVATION ANDFIRST-AID FORFINDS 151
or ‘accretions’ (overlying layers) – which may mask
original detail;- problems relating to organic materials associated
with metals finds – copper corrosion products, for
example, are toxic to some organisms, while lead,
iron and silver corrosion products may result in
the replacement and/or retention of impressions of
organic materials attached to some surfaces.
Other aspects of potential post-excavation materials
degradation are outlined below. Although not compre-
hensive, they are intended to highlight the importance of
‘first-aid’ procedures, based on specific material types.
Inorganic materials: These materials are derived from
non-living things, including ceramics, glass and vitreous
materials; stone; and metals, including metal alloys:
- Ceramics may be susceptible to salt damage,
including the forcing off of outer layers and glazed
surfaces, particularly in low-fired wares. - Glass is particularly prone to deterioration in alka-
line solutions. - Objects made of metals of different types may be
subject to ongoing galvanic corrosion (similar to the
chemical reactions in batteries). They should never,
therefore, be stored together in the same container. - The occurrence of pure gold or silver in burial con-
texts is rare, except, perhaps, in the case of bullion.
Such finds are usually alloyed with other metals to
impart strength and alter working properties dur-
ing manufacture. Some corrosion is likely, therefore,
during burial, with the risk of further corrosion
post-excavation. For these reasons, cleaning, or the
removal of corrosion products, should be undertaken
only as part of full conservation treatment. - Iron tends to corrode outwards, with potential dis-
tortion of shape. In addition, the corrosion prod-
ucts on marine iron are likely to be concreted with
insoluble salts and debris from the burial environ-
ment, sometimes rendering identification difficult. - Finds made of wrought iron (iron ‘worked’ by
hammering, extruding or other mechanical meth-
ods) are likely to corrode slowly along ‘slag inclu-
sion lines’ formed originally during manufacture. If
this process has not reached an end-point during
burial (i.e. if there is remaining metal core), then
such objects are likely to be unstable. Wrought iron
was eventually replaced for many purposes, such as
for making guns and anchors, by cast iron (manu-
factured by heating and then pouring into pre-
shaped moulds). - Cast iron corrodes by a process known as ‘graphi-
tization’, usually leaving behind a metal core, which
may be retained in a potentially reactive state.
Cast-iron in marine environments may be inherently
chemically unstable, with associated exothermic
reactions (giving out heat) leading to the forcing off
of surface layers and possible rapid disintegration,
with objects in some instances becoming hot, or even,
on very rare occasions, exploding when exposed
to air.- Active corrosion in iron objects may be indicated by
‘flash-rusting’, a term applied to rapid oxidation, indi-
cated by the development of spots of bright orange
corrosion products and/or trails of corrosion in
storage water. - Copper (rarely found in a pure state) and copper
alloy finds often corrode in layers, with associated
potential for the preservation of original surfaces
and attached organics. However, they may also
become covered by masking layers, sometimes
concreted to underlying surfaces and should be
cleaned, therefore, only as part of full conservation
treatment. - Lead, tin and associated alloys (such as pewter) are
not always as chemically stable as often thought,
whether wet or dry, being particularly susceptible
to corrosion in the presence of organic acids. They
may also retain evidence of applied surfaces (such
as plating), stamps or markings, which may be
revealed in the X-radiography of thin objects.
Some finds recovered from underwater burial envir-
onments are often referred to as ‘concretions’ due to the
development of thick surface overgrowths that mask the
shape of the contents (figure 16.2). Metal finds (particu-
larly iron) may develop overlying conglomerates, which
may expand to incorporate complexes of objects of
several materials together. This often renders contents
unidentifiable, but considerable detail can be obtained
through the use of X-radiography and appropriate con-
servation treatment (figures 16.3 and 16.4).Organic materials: These materials are those derived
from living things, including: wood; skins and leather;
textiles; bone, horn, ivory and related materials. Burial
may lead to physical weakening and chemical decay,
with much of the original being supported by water. The
evaporation of water post-excavation is likely to result in
shrinkage, loss of shape, cracking, warping and, in some
circumstances, complete disintegration (plate 16.1).Composite artefacts: These are objects made of
more than one material, often incorporating both
organic and inorganic materials, which may pose specific
conservation issues requiring discussion with team
members.